There is no "Congressional Requirement" that has any force of law. POTUS simply acts and it becomes de facto certification. State reports to POTUS.
Once the money is allocated, nothing short of impeachment can stop POTUS. POTUS simply ignores the restriction.
It's ALWAYS been that way except in cases where the Executive chooses to show Congress the courtesy of sending them a letter.
Obama did nothing illegal unless Congress can prove a High Crime or Misdemeanor in an Impeachment proceeding.
Bad on Congress for trusting a Jihadi sympathizer. This is a completely constitutional move by Obama...but he certainly dissed Congress by not even giving them a "reach around" in the form of a letter.
Therein lies the entire point.
Congress states they require certification from the SoS for the funds to be spent...SoS says POTUS is going ahead anyway.